The heating and cooling is very complex process for winding in large power transformer, which involves fluid dynamic, manufacture technology and numerical heat transfer and other related disciplines. Recently, the electromagnetic-Fluid-thermal coupled method and thermal network method are widely used by domestic and foreign researchers, whereas the oil flow distribution can not be taken into account using the two mentioned methods, thus the accuracy of the heat transfer properties will be affected. This paper presents a three-dimensional (3D) electromagnetic-fluid-thermal coupling approach to compute the hot-spot temperature-rise of winding area for large power transformer. An ODFS-400 MVA / 500 kV single-phase auto-transformer is used as the prototype model being studied and the 3D fine computation model of the winding area is established. To validate the correctness and applicability of the proposed method, the numerical results obtained from the proposed method are compared with those of experimental ones.